This invention generally relates to the field of communication systems and, more particularly, to compensating for received signal delays in positioning radio receivers.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have changed the ways in which people communicate. One survey indicates that about 80% of the people who purchase mobile communication units and service subscriptions do so to enhance their personal security. Presumably, many of these subscribers would expect to use their mobile units to aid them in urgent situations, e.g., when their vehicle has become disabled or in an emergency situation requiring rapid medical and/or police response. In these circumstances it would be desirable that the radiocommunication system be able to determine a location of the mobile unit, particularly in the case where the subscriber does not know his or her precise location. Moreover, it is expected that the FCC will soon require that network operators forward the position of an emergency caller to the emergency service provider.
There are many techniques available to generate mobile unit position information. In one positioning system, the mobile unit could estimate its own position and send a message with its coordinates when placing an emergency call. This could be accomplished by, for example, providing the mobile unit with a Global positioning System (GPS) receiver that receives location information from the GPS satellite network. The mobile unit can then transmit this information to the system, which would then forward it to the emergency service provider. This requires, however, significant modification of existing mobile units to include GPS receivers, as well as additional signalling between the mobile units and base stations.
Alternatively, the base stations which transmit signals to, and receive signals from, the mobile units could be used to determine the mobile unit""s location. Various techniques, including attenuation of a mobile unit""s signal, angle-of-arrival, and difference between the time-of-arrival (TDOA) of a mobile unit""s signal at different base stations, have been suggested for usage in providing mobile unit location information. See, for example, the article entitled xe2x80x9cTime Difference of Arrival Technology for Locating Narrowband Cellular Signalsxe2x80x9d by Louis A. Stilp, SPIE Vol. 2602, pp. 134-144. Another system used for locating mobile units in radiocommunication systems is an adjunct system that operates independent of the radiocommunication system. The adjunct system includes its own base stations for locating the mobile unit. The adjunct system may, however, share various components (e.g., an antenna) with the radiocommunication system but processes signals separately. This may be advantageous, for example, as an expedient solution to providing mobile unit location without modifying the large number of existing base stations in a system.
For determining the position of a target mobile unit, some positioning algorithms rely on received signal time-of-arrivals reported from three positioning radio receivers at three or more locations. By processing the time-of-arrivals, the position of the target mobile unit is determined by way of a known constant-radius-circle position-determination algorithm. This algorithm relies on the point of intersection of at least three circles with radiuses corresponding to the time-of-arrivals reported from the positioning receivers.
In practice, a positioning radio receiver, however determines a detection time corresponding to the time when the received signal from the target mobile unit is detected, rather than the time when the received signal actually arrives at the positioning radio receiver. The detection time is the time at which the received signal emerges from the receiver""s detector, having made its way through the receiver""s various RF and IF stages, which add signal delays to the detection of the received signal. Under ideal conditions, the distinction between detection time and arrival time would be insignificant. Because each receiver in a given positioning system would introduce the same amount of delay, which could be readily removed by the system""s position-determination algorithm. Due to component tolerances, however, the assumption that the various positioning receivers will have equal delay is not valid.
For example, certain kinds of IF filters vary greatly in group delayxe2x80x94the time required to propagate energy through the filterxe2x80x94even for nominally identical components. Empirical data suggest that ceramic IF filters, which are widely used in receivers because of their favorable cost/performance characteristics, show group-delay variations of about plus-or-minus 100 nanoseconds from mean. As a result, the determination of position is subject to various uncertainties and tolerances, which result in position inaccuracy and ambiguity. In a positioning system otherwise under ideal conditions, a 100-nanosecond uncertainty in time-of-arrival of the received signal introduces a positional uncertainty of about 30 meters (about 100 feet). Referring to FIG. 1, the position uncertainty caused by plus-or-minus 100 nano second is shown by hashed lines between the crossings of the circles corresponding to time of arrivals of a first positioning radio receiver and a second positioning radio receiver that are used for determining the position of the mobile unit. Each positioning radio receiver is represented by two co-centric circles with radiuses showing the plus-of-minus 100 nano second uncertainty. Thus, positioning radio receivers that have components, e.g., IF filters, drawn from the extremes of tolerance distributions subject a system to positioning errors and ambiguities on the order of several hundred feet. Therefore, there exists a need for a positioning system that compensates for positional uncertainty introduced by component tolerances in positioning radio receivers.
The present invention that addresses this need is exemplified in a positioning system that improves positioning accuracy by measuring and pre-storing signal delays associated with positioning radio receivers. Accordingly, the positioning system for locating the mobile unit according to the present invention includes a detector that detects a received radio signal from the mobile unit by a corresponding positioning radio receiver, and a timing device that determines a detection time associated with the received signal. A processor determines a time-of-arrival for the received radio signal based on the detection time and a measured or computed signal delay that is pre-stored is a storage device. A mobile unit locator processes time-of-arrivals from a plurality of positioning radio receiver to locate the mobile unit.
According to more detailed features of the invention, the signal delay includes a measured or computed group delay associated with a filter included in one of the receiver stages and a measured or computed transmission-line delay associated with a transmission line included in radio receiver. Preferably, the time-of-arrival is based on the difference between the signal delay and the detection time. In an exemplary embodiment, however, the time-of-arrival may be based on the detection time and a delta value derived from the difference of the signal delay relative to a nominal value.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.